The cardiovascular response to submaximal exercise in a chronically instrumented trained dog is a decreased heart rate, increased contractility, and increased end-diastolic ventricular volume. These changes are associated with a coronary blood flow response that is the same before and after training. During the training program, coronary blood flow was found to be reduced and myocardial oxygen consumption the same at similar workloads compared to the untrained condition. These results suggest a biphasic response in the coronary vascular bed to training which may be due to a neurogenic component. Removal of the left stellate ganglion resulted in a completely different response of the heart to training. Atrial pacing and coronary reactive hyperemia were used to demonstrate a potential increase in the coronary vascular bed size. The experiments proposed will determine the coronary vascular responsiveness and the ability to maintain the changes in coronary flow with modification of the exercise program in the instrumented conscious dog. At the termination of the training program, the ability of detraining to modify the results in terms of heart rate and coronary blood flow will also be investigated. The changes in coronary flow with training will be correlted with the changes in the determinants of myocardial oxygen consumption and with substrate utilization of the myocardium. These studies will furnish basic information about the mechanism(s) underlying the change in the coronary vascular bed and their significance with training and detraining. The role of the autonomic nervous system will be further investigated.